It's time to "sticky" this one at the top of the forums again.

If this describes what you're experiencing when lighting off the Cummins on these frosty mornings,.......
Don't worry, it's normal.

The Cummins uses a pair of 12 volt grid heaters in the intake tract
(imagine two large toaster elements glowing bright red)
that preheat incoming air for easier combustion.
The Ford & Chevy use glow plugs inside the combustion chamber.
As the relay cycles the grids on & off, the large amp draw causes the volt meter reading to drop, and the lights to dim when the relay turns on the juice.
The relay timer will turn off the heaters when the intake temps hit a preset limit, or when the vehicle exceeds a preset MPH (usually between 15-25 mph). edit


Don't panic
.........just another of the experiences you can chalk up to being a member of the Cummins Club.